1. Field of the Invention
The present invention relates to the field of manufacture of flat panel displays, and in particular to a device and a method for preprocessing metallic magnesium that is used as a cathode of an OLED.
2. The Related Arts
An organic light-emitting diode or organic light-emitting diode display (OLED) is also referred to as an organic electroluminescent diode, which is a novel displaying technology of which the development was dated back to the middle of the 20th century. The organic electroluminescent diode has various advantages over a liquid crystal display, such as being fully solid state, active emission of light, high brightness, high contrast, being ultra thin, low cost, low power consumption, fast response, wide view angle, wide range of operation temperature, and being capable of flexible displaying. The structure of an organic electroluminescent diode generally comprises a substrate, an anode, a cathode, and an organic function layer and the principle of light emission is that multiple layers of organic materials that are of extremely small thickness is formed between the anode and the cathode through vapor deposition, whereby positive and negative carriers, when injected into the organic semiconductor films, re-combine with each other to generate light. The organic function layer of the organic electroluminescent diode is generally made up of three function layers, which are respectively a hole transport layer (HTL), an emissive layer (EML), and an electron transport layer (ETL). Each of the function layers can be a single layer or more than one layer. For example, the hole transport layer sometimes is further divided into a hole injection layer and a hole transport layer and the electron transport layer may also be divided into an electron transport layer and an electron injection layer. However, they are of substantially the same function and are thus collectively referred to as the hole transport layer and the electron transport layer.
Currently, the manufacture of a full color organic electroluminescent diode is generally done with three methods, which are RGB juxtaposition and individual emission method, white light in combination with color filter method, and color conversion method, among which the RGB juxtaposition and individual emission method is most promising and has the most practical applications. The manufacturing method thereof is that different host and guest light-emitting materials are selected for red, green, and blue colors.
The organic light-emitting diodes can be classified in two types, according to the method of driving, which are active driving and passive driving, namely direct addressing and TFT (Thin-Film Transistor) matrix addressing. The active driving type organic light-emitting diode is the so called active matrix organic light-emitting diode (AMOLED).
The currently adopted technology for small-sized AMOLED display screens is g a low-temperature poly-silicon thin-film transistor (LTPS TFT) backplane carrying a top-emission OLED of which a cathode is formed of a magnesium/silver (Mg/Ag) alloy, where Mg has a work function of −3.68 eV, while Ag has a work function of −4.26 eV, so that electrons are readily injected from the cathode into the electron transport layer. Further, the Mg/AG alloy of 10-20 nm shows excellent transmittance, allowing the light generated by by exciton transition occurring in the emissive layer to transmit out from the interior of the device.
Generally, a metal having a higher work function is more active. For example, lithium (Li) has a work function of −2.1 eV; sodium (Na) has a work function of −2.28 eV; and calcium (Ca) has a work function of −2.9 eV. A metal that is more active can be oxidized more easily. Na needs to be preserved in kerosene and, once contacting air and moisture, will generate reaction, which, if violent, may get flaming and exploded. Thus, although Mg that has a higher work function is chosen for easy use, Mg may still get oxidized in the atmosphere, forming a dense layer of magnesium oxide on the surface thereof.
In a heating and evaporating process occurring in a coating machine, magnesium oxide is released in the form of tiny particles, referred to as “magnesium ash”. The magnesium ash has a very light mass and a large amount of magnesium ash existing in a chamber of the coating machine will contaminate the chamber. Most importantly, they will float and reach a substrate and cause defects on pixels, eventually resulting in dark spots in a light emission zone and affecting the service life and yield rate.